1. Field of the Invention
The present invention is directed to a discharge lamp driving circuit, and more particularly to a circuit for operating a high-pressure gaseous discharge lamp without causing a harmful acoustic resonance.
2. Description of the Prior Art
There has been a growing demand for a discharge lamp operating circuit which is operated at a higher frequency in order to reduce the weight and bulk of the ballasting inductor On the other hand, it is also known that discharge lamps, particularly high-pressure discharge lamps Such is mercury high pressure lamps and sodium vapor lamps suffer from unstable discharge arcs due to "acoustic resonance" when operated at certain high frequencies. U.S. Pat. No. 4,291,254 proposed to select a stable frequency for avoiding such "acoustic resonance". However, such stable frequency is seen only in a limited range and differs from different kinds of lamps, thus reducing the flexibility of circuit design. Further, it is known that an extreme high frequency drive, for example, over 100 KHz may be effective for elimination of the "acoustic resonance", but this eventually results in considerable switching losses and noises which are not acceptable for the lamp operation. To this end, there has been proposed in Japanese Patent Publication (KOKAI) No. 60-262392 to drive the lamp by a composite lamp driving Current having a repeating cycle of an alternating current interrupted by a dc current. This patent is based upon the findinq that the repetitive interruption of the alternating current by the dc current can restrain the occurrence of the "acoustic resonance", even the alternating current is in a frequency range which would otherwise cause the "acoustic resonance". In this sense, this patent is advantageous in selecting a lamp drive frequency without having to consider the "acoustic resonance". Notwithstandinq this advantage, the patent has a certain drawback in that two independent switching circuits, i.e., chopper and inverter circuits, are required for providing the dc current and the alternating current, respectively. This requires duplication of switching elements with consequent complexity in incorporating the respective drivers in circuits for the duplicated switching elements, thus eventually resulting in increased cost and bulk of the physical circuit assembly.